docs: def image_transform_columns(features):
    """Returns a list of columns that prepare_image_transforms() should run on.
  
    Because of beam + pickle, IMAGE_URL_TO_VEC_TRANSFORM cannot be used inside of
    a beam function, so we extract the columns prepare_image_transforms() should 
    run on outside of beam.
    """
    import six
    from trainer import feature_transforms
  
    img_cols = []
    for name, transform in six.iteritems(features):
      if transform['transform'] == feature_transforms.IMAGE_TRANSFORM:
        img_cols.append(name)
  
    return img_cols